This invention relates to water-water tubular heat exchangers in nuclear reactor coolant loops and, more particularly, to heat exchangers designed to transfer core decay heat from nuclear reactors during postulated events where there is a loss of cooling capability via steam generators in the coolant loops of pressurized water reactors. The present invention is particularly useful in passive systems designed to cool a nuclear reactor by natural convection alone.
U.S. Pat. No. 4,753,771 to Conway et al. discloses a well designed passive safety system for a pressurized water nuclear reactor which employs a water-water residual heat removal heat exchanger for removing core decay heat from a reactor core by natural convection. In the Conway et al. passive system, the heat exchanger is disposed in an in-containment reactor water storage tank. The heat exchanger is hydraulically connected via piping to the hot leg and the cold leg of the primary water circuit for cooling the reactor core. In an event of a system failure where, e.g., the coolant pump fails, the coolant water naturally circulates from the reactor core via a pipe through the heat exchanger and then back to the core via another pipe. As disclosed by Convay et al., the heat exchanger generally comprises a plurality of tubes extending from an inlet manifold to an outlet manifold located in the storage tank. Thus, the residual heat removal heat exchanger transfer the decay heat to the water in the storage water tank.
Ideally, a residual heat removal heat exchanger such as the disclosed heat exchanger will rarely, if ever, be needed and the other systems of the plant will perform normally. Nevertheless, residual heat removal heat exchangers contain contaminated primary coolant water at reactor system pressures and, therefore, must be periodically inspected and repaired, if necessary.
Although the heat exchanger disclosed by Conway et al. performs its safety function well, other operational concerns such as routine maintenance and repair of such an exchanger may penalize the on stream operation of the plant. For example, a simple inspection of the tube normally requires that the storage tank be drained before the inspection. However, these tanks are very large (having areas of up to 2000 square feet or more and heights of up to 30 feet or more) and may require many hours to drain. In draining that tank, the removal of the reactor vessel may also be required and thereby further extend the shutdown. Also, this design may have difficulty showing acceptable tank loads following the rupture of the pipe leading to or from the heat exchanger but inside the tank.